A polarizing plate having function of transmitting and shielding light as well as a liquid crystal having switching function of light, serve as a fundamental constitutional element of a display device such as a liquid crystal display (LCD). Application field of this LCD has been expanded from initial small items such as electronic calculator and clock to a wide range of equipments such as portable personal computer, word processor, liquid crystal projector, liquid crystal television set, car navigation system, and indoor and outdoor measurement hardwares. Accordingly, because of use under broad conditions of low temperature to high temperature, low humidity to high humidity, and low light intensity to high light intensity, there has been demanded a polarizing plate having high polarization performance and excellent durability.
At present, a polarizing film is generally prepared by adsorbing or incorporating iodine or a dichromatic dye as a polarizing element in a polarizing film base material such as a stretched and orientated film of polyvinyl alcohol or the derivative thereof or a orientated polyene type film produced by dehydrochlorination of polyvinyl chloride film or dehydration of a polyvinyl alcohol type film. Among them, an iodine type polarizing film using iodine as a polarizing element is excellent in initial polarization performance but weak to water and heat, and has problem in its durability when it is used for a long time under the condition of high temperature and high humidity. In order to improve durability, a method of treating an iodine type polarizing film with formalin or an aqueous solution containing boric acid, or using a polymeric film having a low moisture permeability as a protective film, or the like is considered, but its effect does not seem to be sufficient. On the other hand, a dye type polarizing film using a dichromatic dye as a polarizing element is excellent in humidity resistance and heat resistance but generally not sufficient in initial polarization performance as compared with an iodine type polarizing film.
In neutral color or gray polarizing films produced by adsorbing some kinds of dichromatic dyes in a polymeric film and orientating it, the state (cross position) wherein two polarizing films are superimposed on each other so that their orientation directions can be crossed at right angles, may cause light leakage (color leakage) of a specific wavelength in the wavelength range (400 to 700 nm) of visible light. If such color leakage is caused, when polarizing films are fitted to a liquid crystal panel, in some cases the color of liquid crystal display changes in the dark state. Thus, it is demanded to prevent the color change of liquid crystal display due to color leakage of a specific wavelength in the dark state when polarizing films are fitted to a liquid crystal display device. In order to prevent the color change, in neutral color polarizing film produced by adsorbing some kinds of dichromatic dyes in a polymeric film and orientating it, it is necessary to uniformly lower the transmittance at cross position (cross transmittance) in the wavelength range of visible light.
Furthermore, in the case of a color liquid crystal projection type display, that is, a color liquid crystal projector, a polarizing plate is used for the liquid crystal image-forming part thereof, and formerly there had been used an iodine type polarizing plate having good polarization performance and expressing neutral gray. As mentioned above, an iodine type polarizing plate uses iodine as a polarizer and hence has a problem in that light resistance, heat resistance and wet heat resistance are not sufficient. In order to resolve the problem, a neutral gray polarizing plate using as a polarizer a dye type dichromatic coloring matter has come to be used.
However, in a neutral gray polarizing plate, in order to averagely improve polarization performance and transmittance in the entire wavelength range of visible light, usually coloring matters of three primary colors are used in combination. Therefore, transmittance of light is bad to the requirement of the marketplace that more brightness should be realized as in a color liquid crystal projector, and there is a problem that in order to realize brightness it is necessary to make light intensity at the source higher. Thus, in order to improve transmittance of light, three polarizing plates corresponding to three primary colors, that is, for blue channel, for green channel, and for red channel have come to be used. However, for the reason that light is highly absorbed by polarizing plates, the reason that an image having a small area of 0.9 to 6 inches is expanded to approximately several dozen inches to one hundred and several dozen inches, and the other reasons, reduction of brightness cannot be avoided, and therefore a light source of higher brightness is used. Furthermore, improvement requirement for more brightness of a liquid crystal projector is deep-seated, and consequently light intensity at the source used has grown stronger of itself.
As a dye used for preparation of a dye type polarizing film as mentioned above, there is cited a water-soluble azo compound disclosed in, for example, JP-A-2001-33627, JP-A-2002-220544, JP-A-2002-275381, JP-A-2002-296417, JP-A-2003-64276 or the like.
However, the conventional polarizing plates containing said water-soluble dyes have not yet satisfied the needs of the marketplace sufficiently from the viewpoint of polarization characteristics, absorption wavelength range, hue and the like. Furthermore, among polarizing plates corresponding to three primary colors for a color liquid crystal projector, that is, three polarizing plates for blue channel, for green channel, and for red channel, there are not those which are provided with all of brightness, polarization performance, durability under high temperature or high humidity condition, and light resistance to long time exposure under good balance, and its improvement is demanded.
In addition, Swiss Patent No. 326,553 discloses a trisazo compound having an unsubstituted triazole ring, but does not disclose a trisazo compound having a triazole ring with substituents.